Tim Crowe, Associate Professor in Nutrition, Deakin University states that 'For a healthy adult, if supplements are used, these should normally be taken at levels close to the recommended dietary intake. High-dose supplements should not be taken unless recommended under medical advice.' and backs up the reasoning behind this statement here:
One snippet: 'Food is a complex mix of vitamins, minerals and phytochemicals (plant chemicals). Phytochemicals are an important component of food and help to reduce the risk of conditions such as heart disease, type 2 diabetes and some cancers. Vitamin and mineral supplements do not provide the benefits of phytochemicals and other components found in food, such as fibre.
Vitamin and mineral supplements can’t replace a healthy diet, but a general multivitamin may help if your diet is inadequate or where there is already a well-supported rationale for you to take one. If you feel you could be lacking in certain vitamins and minerals, it is better to look at changing your diet and lifestyle first, rather than reaching for supplements.'